Atherosclerosis is complex vascular disorder involving the interplay between inflammation, coagulation and lipid metabolism. There is increasing evidence that platelet activation but may play important roles in the initiation and/or expansion of atherosclerotic lesions. As platelets contain diverse modulators of inflammation, cell adhesion and endothelial activation, elucidating the mechanisms by which platelets promote atherogenesis may offer novel opportunities for intervention. My group has focused on the involvement of the platelet specific chemokine platelet factor 4 (PF4) in atherosclerosis. We have generated preliminary data in vivo that support the notion that PF4 is proatherogenic. Previous work from our laboratory has defined two receptor dependent pathways that may be responsible for PF4 atherogenicity. First, PF4 inhibits low density lipoprotein receptor (LDLR) dependent low density lipoprotein (LDL) degradation. This results in retention of LDL on the cell surface, which is prone to modification into oxidized LDL (ox-LDL). Second, PF4 activated NF-kB (a transcription factor involved in atherosclerosis and inflammation) via the LDL receptor related protein (LRP). The overriding hypothesis of this proposal is that PF4 activates one or both of these pathways to promote atherosclerotic lesion formation. We further posit that PF4 tetramers oligomerize in the presence of cell surface glycosaminoglycans (GAGs) before activation of the LDLR and/or LRP pathways. To test this hypothesis, we will study the effect of PF4 on atherosclerosis in vivo and elucidate its mechanism of action both in vivo and on vascular cells using in vitro model systems through three related Specific Aims: SA I: "Structural features of PF4 that contribute to proatherogenicity" will further dissect the details of these pathways in vitro, focusing on structural characteristics of PF4. SA II: "Characterize the effects of PF4 on lipoprotein metabolism and atherosclerosis in apoE-/- mice" will expand our characterization of apoE-/- mice lacking PF4, as well as to understand the implications of PF4 overexpression in apoE-/- mice. SA III: "Mechanism of PF4 proatherogenicity in vivo" will examine the importance of both the LDLR and LRP pathways for PF4 mediated atherogenesis. These studies will provide novel insights into the role of PF4, the most abundant protein released by activated platelets, on the development of atherosclerosis, delineate the pathways by which this occurs in vivo, and suggest potential methods to intervene in atherogenesis